U.S. patent application number 16/588024 was filed with the patent office on 2020-08-27 for method and apparatus for performing multi-energy (including dual energy) computed tomography (ct) imaging.
The applicant listed for this patent is Photo Diagnostic Systems,Inc. Invention is credited to Bernard M. Gordon, Olof Johnson, Matthew Len Keeler, William A. Worstell.
Application Number | 20200271597 16/588024 |
Document ID | / |
Family ID | 1000004816196 |
Filed Date | 2020-08-27 |
United States Patent
Application |
20200271597 |
Kind Code |
A1 |
Worstell; William A. ; et
al. |
August 27, 2020 |
METHOD AND APPARATUS FOR PERFORMING MULTI-ENERGY (INCLUDING DUAL
ENERGY) COMPUTED TOMOGRAPHY (CT) IMAGING
Abstract
An improved dual energy CT imaging system for providing improved
imaging and improved material identification.
Inventors: |
Worstell; William A.;
(Wayland, MA) ; Keeler; Matthew Len; (Bolton,
MA) ; Johnson; Olof; (Ashburnham, MA) ;
Gordon; Bernard M.; (Manchester, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Photo Diagnostic Systems,Inc, |
Boxboro |
MA |
US |
|
|
Family ID: |
1000004816196 |
Appl. No.: |
16/588024 |
Filed: |
September 30, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15217742 |
Jul 22, 2016 |
10429323 |
|
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16588024 |
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62196422 |
Jul 24, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B 6/4241 20130101;
G01V 5/0041 20130101; G01N 23/046 20130101; G01N 2223/423 20130101;
A61B 6/4042 20130101; A61B 6/4035 20130101; G01N 2223/401 20130101;
A61B 6/405 20130101; G01N 23/087 20130101; G21K 1/10 20130101; A61B
6/032 20130101; A61B 6/52 20130101; A61B 6/5205 20130101; A61B
6/482 20130101; G01V 5/005 20130101; A61B 6/4233 20130101; G01N
2223/313 20130101 |
International
Class: |
G01N 23/046 20060101
G01N023/046; G01N 23/087 20060101 G01N023/087; A61B 6/03 20060101
A61B006/03; A61B 6/00 20060101 A61B006/00; G21K 1/10 20060101
G21K001/10; G01V 5/00 20060101 G01V005/00 |
Claims
1.-21. (canceled)
22. A system for providing a first energy measurement associated
with a first X-ray energy range produced by a polychromatic X-ray
tube, and a second energy measurement associated with a second
X-ray energy range produced by the same polychromatic X-ray tube,
wherein the second X-ray energy range is different from the first
X-ray energy range, the system comprising: a filtered detector for
detecting X-rays produced by the polychromatic X-ray tube across
the first X-ray energy range and providing an output, wherein the
output of the filtered detector comprises the first energy
measurement; an unfiltered detector for detecting X-rays produced
by the polychromatic X-ray tube across the full X-ray energy range
of the polychromatic X-ray tube and providing an output; and a
processor for determining the second energy measurement, wherein
the second energy measurement is the difference between the output
of the unfiltered detector and the output of the filtered
detector.
23. A system according to claim 22 wherein the first X-ray energy
range comprises higher energy photons and the second X-ray energy
range comprises lower energy photons.
24. A system according to claim 23 wherein the filtered detector
comprises a detector and a filter interposed between the
polychromatic X-ray tube and the detector, and further wherein the
filter is configured to block lower energy photons.
25. A system according to claim 22 further comprising a
polychromatic X-ray tube.
26. A system according to claim 22 wherein an object is disposed
between (i) the polychromatic X-ray tube, and (ii) the filtered
detector and the unfiltered detector.
27. A method for providing a first energy measurement associated
with a first X-ray energy range produced by a polychromatic X-ray
tube, and a second energy measurement associated with a second
X-ray energy range produced by the same polychromatic X-ray tube,
wherein the second X-ray energy range is different from the first
X-ray energy range, the method comprising: providing a system for
providing a first energy measurement associated with a first X-ray
energy range produced by a polychromatic X-ray tube, and a second
energy measurement associated with a second X-ray energy range
produced by the same polychromatic X-ray tube, wherein the second
X-ray energy range is different from the first X-ray energy range:
a filtered detector for detecting X-rays produced by the
polychromatic X-ray tube across the first X-ray energy range and
providing an output, wherein the output of the filtered detector
comprises the first energy measurement; an unfiltered detector for
detecting X-rays produced by the polychromatic X-ray tube across
the full X-ray energy range of the polychromatic X-ray tube and
providing an output; and a processor for determining the second
energy measurement, wherein the second energy measurement is the
difference between the output of the unfiltered detector and the
output of the filtered detector; using the polychromatic X-ray tube
to cause the full X-ray energy range of the polychromatic X-ray
tube to be received by the filtered detector and the unfiltered
detector; and using the filtered detector to determine the first
energy measurement, and using the processor to determine the second
energy measurement.
28. A method according to claim 27 wherein the first X-ray energy
range comprises higher energy photons and the second X-ray energy
range comprises lower energy photons.
29. A method according to claim 28 wherein the filtered detector
comprises a detector and a filter interposed between the
polychromatic X-ray tube and the detector, and further wherein the
filter is configured to block lower energy photons.
30. A method according to claim 27 wherein the system further
comprises a polychromatic X-ray tube.
31. A method according to claim 27 wherein an object is disposed
between (i) the polychromatic X-ray tube, and (ii) the filtered
detector and the unfiltered detector.
Description
REFERENCE TO PENDING PRIOR PATENT APPLICATION
[0001] This patent application claims benefit of pending prior U.S.
Provisional Patent Application Ser. No. 62/196,422, filed Jul. 24,
2015 by Photo Diagnostic Systems, Inc. and William A. Worstell et
al. for METHOD AND APPARATUS FOR PERFORMING DUAL ENERGY COMPUTED
TOMOGRAPHY (CT) IMAGING (Attorney's Docket No. PDSI-1 PROV), which
patent application is hereby incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to imaging systems in general, and
more particularly to computed tomography (CT) imaging systems.
BACKGROUND OF THE INVENTION
[0003] In many situations it can be desirable to image the interior
of an object. By way of example but not limitation, in the medical
field, it can be desirable to image the interior of a patient's
body so as to allow viewing of internal structures without
physically penetrating the skin. By way of further example but not
limitation, in the security field, it can be desirable to image the
interior of a container (e.g., a suitcase, a package, etc.) so as
to allow viewing of internal structures without physically opening
the container.
Computed Tomography (CT)
[0004] Computed Tomography (CT) has emerged as a key imaging
modality in the medical and security fields, among others. CT
imaging systems generally operate by directing X-rays into an
object (e.g., a body or a container) from a variety of positions,
detecting the X-rays passing through the object, and then
processing the detected X-rays so as to build a three-dimensional
(3D) data set, and a 3D computer model, of the interior of the
object (e.g., the patient's anatomy or the contents of the
container). The 3D data set and 3D computer model can then be
visualized so as to provide images (e.g., slice images, 3D computer
images, etc.) of the interior of the object (e.g., the patient's
anatomy or the contents of the container).
[0005] By way of example but not limitation, and looking now at
FIGS. 1 and 2, there is shown an exemplary prior art CT imaging
system 5. CT imaging system 5 generally comprises a torus 10 which
is supported by a base 15. A center opening 20 is formed in torus
10. Center opening 20 receives the object (e.g., the body or the
container) which is to be scanned by CT imaging system 5.
[0006] Looking next at FIG. 3, torus 10 generally comprises a fixed
gantry 25, a rotating disc 30, an X-ray tube assembly 35 and an
X-ray detector assembly 40. More particularly, fixed gantry 25 is
disposed concentrically about center opening 20. Rotating disc 30
is rotatably mounted to fixed gantry 25. X-ray tube assembly 35 and
X-ray detector assembly 40 are mounted to rotating disc 30 in
diametrically-opposing relation, such that an X-ray beam 45
(generated by X-ray tube assembly 35 and detected by X-ray detector
assembly 40) is passed through the object (e.g., the body or the
container) disposed in center opening 20. Inasmuch as X-ray tube
assembly 35 and X-ray detector assembly 40 are mounted on rotating
disc 30 so that they are rotated concentrically about center
opening 20, X-ray beam 45 will be passed through the object (e.g.,
the body or the container) along a full range of radial positions,
so as to enable CT imaging system 5 to create a "slice" image of
the object penetrated by the X-ray beam. Furthermore, by moving the
object (e.g., the body or the container) and/or CT imaging system 5
relative to one another during scanning, a series of slice images
can be acquired, and thereafter appropriately processed, so as to
create a 3D data set of the scanned object and a 3D computer model
of the scanned object.
[0007] In practice, it is now common to effect helical scanning of
the object so as to generate a 3D data set of the scanned object,
which can then be processed to build a 3D computer model of the
scanned object. The 3D data set and/or 3D computer model can then
be visualized so as to provide images (e.g., slice images, 3D
computer images, etc.) of the interior of the object (e.g., the
patient's anatomy or the contents of the container).
Beam Hardening
[0008] In practice, X-ray tube assembly 35 is typically a
polychromatic X-ray source, i.e., X-ray tube assembly 35 typically
comprises an X-ray tube of the sort which emits X-rays with a range
of different energies. However, as the X-ray beam from a
polychromatic X-ray source passes through the object which is being
scanned, low energy X-ray photons are generally attenuated more
easily than high energy X-ray photons. Thus, the X-ray beam from a
polychromatic X-ray source preferentially loses the lower-energy
parts of its spectrum as it passes through the object. This is a
particular problem with high atomic number materials such as bone
and metals which can heavily attenuate the lower-energy parts of
the X-ray beam, and can (among other things) produce image
artifacts between two high attenuation materials (e.g., between two
materials which have high atomic numbers). This phenomenon is
sometimes referred to as "beam hardening". Various algorithms have
been developed in an effort to correct for beam hardening, but such
algorithms generally suffer from the fact that they require certain
assumptions to be made (e.g., that the polychromatic X-ray source
has a fixed spectrum which does not change as the X-ray tube heats
up, that the X-ray absorption spectrum has an idealized shape,
etc.).
Dual Energy CT
[0009] X-ray attenuation is generally caused by (i) the scattering
of radiation by the object which is being scanned, and/or (ii) the
absorption of radiation by the object which is being scanned. The
mechanisms primarily responsible for these two effects are Compton
scatter and photoabsorption. Compton scatter and photoabsorption
vary according to (a) the energy of the photons in the X-rays, and
(b) the composition of the object which is being scanned. For this
reason, it has been recognized that measurements taken at two
different X-ray energies (i.e., "dual energy") can be used to
distinguish between different materials in the object which is
being scanned. These two different measured X-ray energies are not
necessarily restricted to monochromatic measurements, and in
practice the two measurements are taken over broad ranges of low
and high energies (i.e., a low-energy polychromatic measurement is
used as one of the X-ray energies of the dual energy scan, and a
high-energy polychromatic measurement is used as the other of the
X-ray energies of the dual energy scan).
[0010] Several different approaches can be used to generate
measurements of two different X-ray energy ranges.
[0011] First, two different X-ray spectra can be applied to the
object which is being scanned, and an unfiltered detector can be
used to detect the differences in attenuation by the object which
is being scanned. Accordingly, with some dual energy CT imaging
systems, the system is provided with a single X-ray tube with rapid
switching between two different voltages, and an unfiltered
detector is used to detect the differences in attenuation by the
object which is being scanned with two different X-ray voltages.
However, this approach suffers from relatively high cost, since it
requires that the single X-ray tube be driven by two different
voltages, which generally requires the provision of either two
high-voltage power supplies or a single high-speed switching power
supply. With other dual energy CT imaging systems, the system is
provided with two X-ray tubes driven at different voltages (whereby
to apply two different X-ray spectra to the object which is being
scanned), and an unfiltered detector is used to detect the
differences in attenuation by the object which is being scanned by
the two different X-ray tubes/two different X-ray tube voltages.
However, this approach also suffers from relatively high cost,
since it requires the provision of two X-ray tubes.
[0012] Second, it has also been recognized that a single X-ray
spectra can be applied to the object which is being scanned, and
then a dual-filter detector (i.e., a detector having two different
X-ray spectrum filters) can be used to obtain measurements over two
different X-ray energy ranges (e.g., a "high" X-ray energy band and
a "low" X-ray energy band), whereby to detect the differences in
attenuation by the object which is being scanned. Accordingly, with
some dual energy CT imaging systems, the system is provided with a
single X-ray tube driven by a single voltage, and a dual-filter
detector is used to obtain measurements over two different X-ray
energy ranges, i.e., one filter is configured to maximize the
detection of higher energy photons and one filter is configured to
maximize the detection of lower energy photons. However, this
approach suffers from relatively inefficient use of the available
photons, because each filter must be something of a compromise in
determining what fraction of the photons of the "undesired"
spectrum must be allowed to pass in order to get some portion of
the photons of the "desired" spectrum. Stated another way, the two
filters used in the dual-filter detector cannot cut off at an
infinite rate, therefore, the design of each filter must be
something of a compromise.
[0013] Again, where the X-ray source is a polychromatic X-ray
source (i.e., the X-ray tube assembly emits X-rays with a range of
different energies), issues arise due to beam hardening (i.e., the
preferential attenuation of low energy X-ray photons by the object
being scanned). This can create issues in material identification.
Again, various algorithms have been developed in an effort to
correct for beam hardening, but such algorithms generally suffer
from the fact that they require certain assumptions to be made
(e.g., that the polychromatic X-ray source has a fixed spectrum
which does not change as the X-ray tube heats up, that the X-ray
absorption spectrum has an idealized shape, etc.).
[0014] Thus there is a need for an improved multi-energy (including
dual energy) CT imaging system providing improved imaging (e.g.,
images free of beam hardening artifacts or approximations) and
improved material identification.
SUMMARY OF THE INVENTION
[0015] These and other objects of the present invention are
addressed by the provision and use of an improved multi-energy
(including dual energy) CT imaging system providing improved
imaging (e.g., images free of beam hardening artifacts or
approximations) and improved material identification.
[0016] In one preferred form of the invention, there is provided a
novel approach for performing dual energy CT imaging.
[0017] First, the object is scanned using a CT imaging system
comprising (i) an X-ray tube assembly comprising a single X-ray
tube driven at a single voltage and producing a polychromatic X-ray
beam, and (ii) an X-ray detector assembly wherein each detector is
capable of measuring two different X-ray energy ranges, i.e., a
"high energy measurement" and a "low energy measurement". This may
be done using conventional "dual-filter" detectors or using a novel
"single-filter" detector as described herein. In either case, such
scanning produces two polyenergetic signals, g.sub.HIGH ("high
energy") and g.sub.LOW ("low energy"), for each rotational position
of the X-ray tube assembly/X-ray detector assembly about the object
which is being scanned, i.e., for each "line of response". In
general, additional filters (or their functional equivalent) may be
used to obtain additional polyenergetic signals which can be used
to further supplement the available spectral information, e.g.,
appropriate filtering may be provided to generate three or more
polyenergetic signals for enabling multi-energy CT.
[0018] Next, for each line of response, the ratio
g.sub.HIGH/g.sub.LOW (or "R") is computed.
[0019] R and g (i.e., g.sub.HIGH or g.sub.LOW) are then used as the
indices for an appropriate lookup table to obtain multiplicative
factors ("A" or "B", respectively) to transform the polyenergetic
data g (i.e., g.sub.HIGH or g.sub.LOW, respectively) into the
predicted signal for a purely monochromatic system of any
monoenergy. The monoenergetic sinogram (G.sub.ENERGY) comprises the
collection of monoenergetic signals for any single energy
(subscript ".sub.ENERGY") at every line of response including the
lines of response associated with both highly filtered (g.sub.HIGH)
and lesser filtered or unfiltered (g.sub.LOW) detectors. Note that
the function R need not be the specific ratio g.sub.HIGH/g.sub.LOW,
or any other ratio for that matter, but may be any function which
is sensitive to the differential response of the two (or more)
different kinds of detector channels g.sub.HIGH and g.sub.LOW.
[0020] A lookup table (comprising the appropriate multiplicative
factors "A.sub.ENERGY.sup." and "B.sub.ENERGY.sup.") can be
generated to produce a corresponding monochromatic sinogram
G.sub.ENERGY, and from there, a corresponding image for any single
monoenergy (subscript ".sub.ENERGY"). By way of example but not
limitation, a lookup table (comprising the appropriate
multiplicative factors "A.sub.160" and "B.sub.160") can be
generated to produce a corresponding monochromatic sinogram
G.sub.160 (i.e., a corresponding monochromatic sinogram at
monoenergy 160 kev), and from there, a corresponding image
I.sub.160 for that monoenergy (i.e., a corresponding image for that
monoenergy 160 kev). By way of further example but not limitation,
a lookup table (comprising the appropriate multiplicative factors
"A.sub.40" and "B.sub.40") can be generated to produce a
corresponding monochromatic sinogram G.sub.40 (i.e., a
corresponding monochromatic sinogram at monoenergy 40 kev), and
from there, a corresponding image I.sub.40 for that monoenergy
(i.e., a corresponding image for that monoenergy 40 kev).
[0021] Significantly, these "synthetic" monoenergetic images (e.g.,
I.sub.160, I.sub.40, etc.) are free of beam hardening
artifacts.
[0022] Furthermore, material composition can be obtained by a
suitable function operating on one or more monoenergetic images
(e.g., I.sub.160, I.sub.40, etc.).
[0023] For example, a monoenergetic ratio image I.sub.R can be
produced and used to determine (i) the effective atomic number
(Z.sub.eff) for each voxel in the fused image, and (ii) the
electron density (Rho) for each voxel in the fused image, so that
the material composition of the object can be determined. By way of
example but not limitation, I.sub.R may be I.sub.160/I.sub.40.
[0024] In general, the signals g.sub.x associated with the
differently filtered detectors, in conjunction with the function R,
can be transformed to an equivalent signal assuming a monoenergetic
X-ray source. This transformation may be a simple mathematical
function, but in most instances a lookup table is the preferred
embodiment because of the computational savings available by using
a lookup table. In addition, the transformation from a
polyenergetic signal to an equivalent monoenergetic signal may be
empirically derived, in which case a table-based implementation is
preferred. This method of correcting for beam hardening is, in
principle, exact, and does not suffer from the approximations
present in other methodologies.
[0025] The monoenergetic images, being free of beam-hardening, can
be used to characterize different aspects of the materials being
scanned. In one implementation, a large number of monoenergetic
images can be assembled to produce a virtual "X-ray spectrometer"
where each voxel of the assembled image has the full absorption
spectrum. In the opposite extreme, it may be useful to simply
predict the absorptive properties of materials at a single
monoenergy. For example, this method can be used to predict the
absorption of high energy gamma rays used in nuclear imaging
(Single Photon Emission Computed Tomography "SPECT" scanning uses
the radioactive element Technetium-99, which emits monochromatic
140 kev radiation). In the preferred embodiment, two monoenergetic
images (e.g., I.sub.160 and I.sub.40) are transformed into a
monoenergetic ratio image I.sub.R whose values are proportional to
the effective atomic number of the scanned material. Other material
or physical properties can be derived from monochromatic images
(e.g., I.sub.160 and I.sub.40) such as electron density (Rho) or
degree of Compton scattering.
[0026] In one preferred form of the invention, there is provided a
multi-energy computed tomography (CT) imaging system for providing
an image of an object, the system comprising:
[0027] a polychromatic X-ray source;
[0028] a detector for detecting X-rays from the polychromatic X-ray
source after the X-rays have passed through an object and for
providing a first set of polychromatic energy measurements relating
to a first polychromatic X-ray spectrum passed through the object
and for providing a second set of polychromatic energy measurements
relating to a second polychromatic X-ray spectrum passed through
the object; and
[0029] a processor configured to: [0030] (i) transform at least one
of the first set of polychromatic energy measurements and the
second set of polychromatic energy measurement into a corresponding
first monochromatic data set associated with X-rays at a selected
first monochromatic energy level and into a corresponding second
monochromatic data set associated with X-rays at a selected second
monochromatic energy level; [0031] (ii) transform the first
monochromatic data set into a first monochromatic image and
transform the second monochromatic data set into a second
monochromatic image; and [0032] (iii) use at least one of the first
monochromatic image and the second monochromatic image to perform
at least one of (a) provide an image free from beam hardening
artifacts, and (b) provide identification of material properties
within the object.
[0033] In another preferred form of the invention, there is
provided a method for providing an image of an object free from
beam hardening artifacts and/or providing identification of
material properties of the object, the method comprising:
[0034] providing a first set of polychromatic energy measurements
relating to a first polychromatic X-ray spectrum passed through the
object and providing a second set of polychromatic energy
measurements relating to a second polychromatic X-ray spectrum
passed through the object;
[0035] transforming at least one of the first set of polychromatic
energy measurements and the second set of polychromatic energy
measurement into a corresponding first monochromatic data set
associated with X-rays at a selected first monochromatic energy
level and into a corresponding second monochromatic data set
associated with X-rays at a selected second monochromatic energy
level;
[0036] transforming the first monochromatic data set into a first
monochromatic image and transforming the second monochromatic data
set into a second monochromatic image; and
[0037] using at least one of the first monochromatic image and the
second monochromatic image to provide at least one of (a) an image
free from beam hardening artifacts, and (b) identification of
material properties within the object.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] These and other objects and features of the present
invention will be more fully disclosed or rendered obvious by the
following detailed description of the preferred embodiments of the
invention, which is to be considered together with the accompanying
drawings wherein like numbers refer to like parts and further
wherein:
[0039] FIGS. 1 and 2 are schematic views showing the exterior of an
exemplary prior art CT imaging system;
[0040] FIG. 3 is a schematic view showing various components in the
torus of the exemplary prior art CT imaging system shown in FIGS. 1
and 2;
[0041] FIG. 4 is a schematic view showing components in the torus
of a novel multi-energy (including dual energy) CT imaging system
formed in accordance with the present invention;
[0042] FIG. 5 is a schematic view of a dual-filter detector which
may be utilized in the X-ray detector assembly of the novel
multi-energy (including dual energy) CT imaging system shown in
FIG. 4;
[0043] FIG. 6 is a schematic view of a single-filter detector which
may be utilized in the X-ray detector assembly of the novel
multi-energy (including dual energy) CT imaging system shown in
FIG. 4;
[0044] FIG. 7 is a schematic representation showing how
polyenergetic data may be transformed to monoenergetic data;
[0045] FIG. 8 is a schematic representation showing how multiple
monoenergetic data sets and images may be produced, and multiple
monoenergetic images may be transformed into an image conveying
material properties;
[0046] FIG. 9 is a schematic view showing how polyenergetic data
sets may be transformed into any number of monoenergy data sets,
and how two or more monoenergetic images may be transformed into an
image conveying material properties; and
[0047] FIG. 10 is a table showing how raw polychromatic data may be
transformed into equivalent monochromatic data.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0048] In accordance with the present invention, there is provided
a novel multi-energy (including dual energy) CT imaging system
providing improved imaging (e.g., images free of beam hardening
artifacts or approximations) and improved material
identification.
Apparatus for Performing Dual Energy Computed Tomography (CT)
Imaging
[0049] In accordance with the present invention, and looking now at
FIG. 4, there is provided a novel multi-energy (including dual
energy) CT imaging system 105. Multi-energy (including dual energy)
CT imaging system 105 is substantially the same as the exemplary
prior art CT imaging system 5 previously discussed, except as will
hereinafter be discussed.
[0050] More particularly, in the preferred form of the present
invention, multi-energy (including dual energy) CT imaging system
105 generally comprises a torus 110 which is supported by a base
115. A center opening 120 is formed in torus 110. Center opening
120 receives the object (e.g., the body or the container) which is
to be scanned by multi-energy (including dual energy) CT imaging
system 105.
[0051] Still looking now at FIG. 4, torus 110 generally comprises a
fixed gantry 125, a rotating disc 130, an X-ray tube assembly 135
and an X-ray detector assembly 140. More particularly, fixed gantry
125 is disposed concentrically about center opening 120. Rotating
disc 130 is rotatably mounted to fixed gantry 125. X-ray tube
assembly 135 and X-ray detector assembly 140 are mounted to
rotating disc 130 in diametrically-opposing relation, such that an
X-ray beam 145 (generated by X-ray tube assembly 135 and detected
by X-ray detector assembly 140) is passed through the object (e.g.,
the body or the container) disposed in center opening 120.
[0052] In one preferred form of the invention, X-ray tube assembly
135 comprises a polychromatic X-ray tube assembly, i.e., X-ray tube
assembly 135 emits X-rays with a range of different energies. In
one preferred form of the invention, X-ray tube assembly 135
comprises a single X-ray tube which is driven by a single
voltage.
[0053] Inasmuch as X-ray tube assembly 135 comprises a single
polychromatic X-ray tube driven by a single voltage, in order to
allow the imaging system to be used for multi-energy (including
dual energy) CT imaging, it is necessary for X-ray detector
assembly 140 to provide, for each of its detectors, measurements at
two or more different X-ray energy ranges, e.g., a "high energy
measurement" which maximizes the detection of higher energy photons
and a "low energy measurement" which maximizes the detection of
lower energy photons, with or without additional energy
measurements.
[0054] For clarity of description, multi-energy (including dual
energy) CT imaging system 105 will generally hereinafter be
discussed in the context of a dual energy CT imaging system,
however, it should be appreciated that the CT imaging system may
utilize three or more energy measurements without departing from
the scope of the present invention.
[0055] If desired, this may be accomplished in the manner of the
prior art, i.e., by providing a dual-filter detector such as the
dual-filter detector shown in FIG. 5. More particularly, in this
form of the invention, each detector 150 (FIG. 5) of X-ray detector
assembly 140 is provided with two separate detection regions 155,
160, with detection region 155 being provided with a first filter
165 which is configured to maximize the detection of higher energy
photons (whereby to provide the "high energy measurement" used for
dual energy CT scanning), and with detection region 160 being
provided with a second, different filter 170 which is configured to
maximize the detection of lower energy photons (whereby to provide
the "low energy measurement" used for dual energy CT scanning).
However, this prior art approach has the drawback of increased cost
and lower photon yield, particularly with respect to the "low
energy measurement" since the "low energy measurement" is
restricted to low energy photons and such low energy photons are
more heavily attenuated as they encounter the object being scanned,
thereby yielding lower photon yields.
[0056] For this reason, the present invention provides an improved
approach for providing, for each of its detectors, measurements at
two different X-ray energy ranges (i.e., a "high energy
measurement" and a "low energy measurement") in order to enable
dual energy CT scanning.
[0057] More particularly, and looking now at FIG. 6, with the
improved approach of the present invention, each detector 150 of
X-ray detector assembly 140 is provided with two separate detection
regions 155, 160. Detection region 155 is provided with a filter
165 which is configured to maximize the detection of higher energy
photons (whereby to provide the "high energy measurement" used for
dual energy CT scanning). Detection region 160 is not provided with
a filter (hence, the detector shown in FIG. 6 may be considered to
be a "single-filter detector", rather than a "dual-filter detector"
such as is shown in FIG. 5).
[0058] As a result of this construction, detection region 155 will
provide a measurement "X" which is representative of the higher
energy portion of the X-ray spectrum passing through the object
which is being scanned, and detection region 160 will provide a
measurement "Y" which is representative of the total X-ray spectrum
passing through the object which is being scanned. The present
invention recognizes that if the value of the aforementioned
measurement "X" is subtracted from the value of the aforementioned
measurement "Y", the resulting value "Z" will be representative of
the lower energy portion of the X-ray spectrum passing through the
object which is being scanned. Thus, with this approach, by
measuring the high energy photons passing through filter 165 and
striking detection region 155 (i.e., the aforementioned measurement
"X"), the "high energy measurement" used for dual energy CT
scanning can be obtained. And by measuring the total X-ray spectrum
striking detection region 160 (i.e., the aforementioned measurement
"Y"), and then subtracting the value of the higher energy portion
of the X-ray spectrum striking detection region 155 (i.e., the
aforementioned measurement "X"), the "low energy measurement"
(i.e., the aforementioned measurement "Z") can be obtained. In this
way, the "single-filter detector" shown in FIG. 6 can be used to
acquire the "high energy measurement" and the "low energy
measurement" used for dual energy CT scanning.
[0059] Alternatively, a proxy for the "high energy measurement" can
be taken as the signal striking region 160, as its average spectral
response represents a higher energy than that of region 155, and
the "low energy measurement" can be taken as the signal striking
region 155, since filter 165 will ensure that those photons
striking region 155 will be at a lower energy level than the
photons striking region 160.
Method for Performing Multi-Energy (Including Dual Energy) Computed
Tomography (CT) Imaging
[0060] In accordance with the present invention, there is also
provided a new process for using the measurements taken at two or
more different polychromatic X-ray energies to provide multi-energy
(including dual energy) CT imaging.
[0061] For clarity of description, multi-energy (including dual
energy) CT imaging system 105 will generally hereinafter be
discussed in the context of a dual energy CT imaging system,
however, it should be appreciated that the CT imaging system may
utilize three or more energy measurements without departing from
the scope of the present invention.
[0062] The process described below shows how polychromatic dual
energy data can be processed into synthetic monochromatic images
which are both free of beam hardening artifacts and which contain
information on material composition.
[0063] More particularly, when dual energy CT imaging is to be
performed on an object (e.g., a body or a container), the object is
placed in center opening 120 of novel dual energy CT imaging system
105, rotating disc 130 is rotated about fixed gantry 125, X-ray
tube assembly 135 is energized so as to emit polychromic X-ray beam
145, and X-ray detector assembly 140 is operated so as to collect
two energy measurements for each detector of X-ray detector
assembly 140, i.e., a polychromatic "high energy measurement" and a
polychromatic "low energy measurement".
[0064] As discussed above, and as shown in FIG. 5, the two energy
measurements may be obtained for each detector by using a
dual-filter detector where two different filters are positioned
over each detector 150, i.e., by positioning filter 165 over
detection region 155 and by positioning filter 170 over detection
region 160. In this way, the "high energy measurement" is obtained
from the output of detection region 155 and the "low energy
measurement" is obtained from the output of detection region
160.
[0065] Alternatively, and more preferably, and as also discussed
above, the two energy measurements may be obtained for each
detector of X-ray detector assembly 140 by positioning a filter
over half of the detector and leaving the other half of the
detector exposed (i.e., by positioning filter 165 over detection
region 155 and leaving detection region 160 unfiltered, in the
manner shown in FIG. 6). In this way, the polychromatic "high
energy measurement" may be obtained from the output of detection
region 155 and a polychromatic "low energy measurement" may be
obtained from the the output of detection region 160.
[0066] These two polychromatic energy measurements are made for
each detector of X-ray detector assembly 140 for each rotational
position of rotating disc 130 about fixed gantry 125, i.e., for
each "line of response" of the X-ray beam passing through the
object which is being scanned), thereby yielding two polyenergetic
sinograms, g.sub.HIGH ("high energy") and g.sub.LOW ("low
energy").
[0067] Then, for each line of response, the ratio
g.sub.HIGH/g.sub.LOW (or "R") is computed.
[0068] The signal for any given line of response, whether
associated with a filtered detector or an unfiltered detector
(i.e., g.sub.HIGH or g.sub.LOW, respectively), can be converted to
the predicted signal for a purely monochromatic system of ANY
monoenergy. This is achieved using a lookup table where R and g
(either g.sub.HIGH or g.sub.LOW) are used as the lookup indices to
obtain the appropriate multiplier to transform the CT-acquired
polyenergetic sinogram g (either g.sub.HIGH or g.sub.LOW) into a
synthetic monoenergetic sinogram G.sub.ENERGY, where ".sub.ENERGY"
may be any monoenergetic level (e.g., G.sub.160 which represents
the synthetic monoenergetic sinogram at a monoenergy of 160 kev,
G.sub.40 which represents the synthetic monoenergetic sinogram at a
monoenergy of 40 kev, etc.). In practice, because the response of
the filtered and unfiltered channels are different, separate tables
and separate lookup values are required for each of the filtered
and unfiltered channels (i.e., the multipliers "A.sub.ENERGY" and
"B.sub.ENERGY" respectively to produce a corresponding
monochromatic sinogram of G.sub.ENERGY). In this way, synthetic
monoenergetic signal levels are predicted for every line of
response, generating a full resolution monoenergetic synthetic
sinogram. This may be done for any monoenergy using the appropriate
multipliers.
[0069] Next, CT reconstruction techniques are applied to
monoenergetic sinograms to produce monoenergetic images, i.e.
I.sub.ENERGY. The monoenergetic images may be produced for any
monoenergy, e.g., I.sub.160 which is the synthetic monoenergetic
image at a monoenergy of 160 kev, I.sub.40 which is the synthetic
monoenergetic image at a monoenergy of 40 kev, etc. Any number of
monoenergetic images can be generated from the data acquired using
the methods described. These monoenergetic images are free of beam
hardening artifacts and are therefore potentially useful for that
reason alone. By way of example but not limitation, a single
monoenergetic image could be potentially very useful for "radiation
planning". In radiation therapy it is useful to know how the
radiation used to target and kill tumors attenuates as the
radiation beam travels through the body. A monoenergetic image can
be generated at the same energy as the radiation used for therapy
purposes. Each location within the image (i.e., each voxel)
contains the predicted attenuation properties of the material for
the therapy beam.
[0070] By way of example but not limitation, and looking now at
FIG. 7, the top of the figure shows a representation of
polychromatic dual energy data. In the field of tomographic
imaging, raw data commonly takes on the form of a "sinogram" or
"fanogram". The data on the left side of FIG. 7 are in sinogram
format. Each "detector channel" consists of several filtered
detectors. For example, with a dual energy system, there are two
polyenergetic signals which are measured, g.sub.HIGH and g.sub.Low
(for high and low polyenergetic ranges, respectively). The insert
shows example numerical data for several detector pairs. This raw
data could be turned into an uncorrected polyenergetic image using
any number of industry-standard reconstruction techniques including
filtered back projection. The polyenergetic image would suffer from
beam-hardening image artifacts. The middle row of FIG. 7 shows an
example multiplier map which transforms the raw polyenergetic data
into monoenergetic data. The multiplier for each detector channel
has two components, A and B, to convert the polyenergetic signals
g.sub.HIGH and g.sub.Low respectively. The bottom row of FIG. 7
shows the resulting monoenergetic data. The two polyenergetic
signals g.sub.HIGH and g.sub.Low are converted to a monoenergetic
signal g.sub.ENERGY, where ".sub.ENERGY" may be any monoenergetic
level (e.g., G.sub.160 which represents the synthetic monoenergetic
sinogram at a monoenergy of 160 kev, G.sub.40 which represents the
synthetic monoenergetic sinogram at a monoenergy of 40 kev, etc.).
Monoenergetic data does not suffer from beam hardening artifacts
when turned into an image.
[0071] Synthetic monoenergetic images also contain information on
the material composition along each line of response. For example,
a mathematical function rho(I.sub.HIGH, I.sub.LOW) could be used to
directly determine the electron density of the material at that
location (i.e., voxel).
[0072] From a computational efficiency point of view, the preferred
embodiment uses a lookup table to transform the monoenergetic
values at each location within the image into a material value (in
this example, rho).
[0073] The underlying physical interactions which determine the
attenuation of X-rays are generally characterized by Compton
scattering (interacting with loosely-bound electrons) and the
photoelectric effect (resonant interactions with electrons in
atomic shells). Any physical property strongly dependent on the
measured strength of these two kinds of interactions can be
evaluated. For example, the mass-density of a material, largely
dependent on the kind of atoms the material is composed of (a
function of the photoelectric effect) and the number of atoms per
volume (a function of the electron density), can be determined
using the multiple synthetic monoenergetic images I.sub.HIGH,
I.sub.LOW (which could be, for example, I.sub.160, I.sub.40).
[0074] Looking now at FIG. 8, although any number of monoenergetic
datasets and images can be made, in the preferred embodiment, three
monochromatic images are made. The mid-energy image optimizes the
signal/noise ratio and is free of beam-hardening artifacts. The low
and high-energy monochromatic images are transformed to produce an
image of the material atomic composition.
Generation of Lookup Tables for Monoenergetic Sinograms
[0075] The conversion between polyenergetic sinogram data into
monoenergetic sinogram data has been described above as a
mathematical function, with the preferred embodiment generalized to
a lookup table. The mathematical function and/or lookup table
contents could be determined entirely from theoretical computations
or from empirical data. For both approaches, the goal is to
transform the polyenergetic signals measured at the detectors to
the expected monoenergetic signals. To verify that the transform
operation produces the "expected" monoenergetic signals, it is
usually convenient to select test objects of simple shape and
composition. In most embodiments, the "expected" values are the
best scientifically derived values. In our present embodiment,
these target values are obtained from data published by the
National Institute of Standards and Technology (NIST), but could be
obtained from any source.
[0076] Using a completely theoretical approach, one would model the
spectrum of the X-ray source, and compute the expected signal level
for the different kinds of filtered detectors. The signal levels
for the various filtered detectors would be computed a second time
assuming a monoenergetic X-ray source. Taking the ratio of the
computed detector signals for the monoenergetic X-ray source to the
computed detector signals for the polyenergetic source would
determine the multipliers necessary to transform a measured
polyenergetic signal to the target monoenergy signal.
[0077] Using a completely empirical approach would involve making
measurements of many objects (with varying composition and
thickness) using a standard broad-spectrum (i.e., polychromatic)
X-ray source. The equivalent measurements would be acquired using a
monoenergetic source. By comparing values, one could determine the
multiplicative values (e.g., "A.sub.ENERGY" and "B.sub.ENERGY") to
translate one set of measurements (i.e., the polychromatic
measurements) to another set of measurements (i.e., the
monochromatic measurements at the monoenergetic energy level
"ENERGY", e.g., 160 kev, 40 kev, etc.). However, it will be
appreciated that a purely empirical approach is challenging because
generating monochromatic X-rays is generally difficult, and because
the number measurements would be large.
[0078] A practical approach, and the preferred embodiment for the
present invention, involves a predominantly theoretical approach
which is fine-tuned to match empirical measurements. Known material
samples with varying thicknesses can be scanned using a
polyenergetic X-ray source to generate polyenergetic data, and then
the polyenergetic data can be transformed to corresponding
monoenergetic data using the derived lookup tables. The
monoenergetic detector response to these known samples are easily
computed, and can be compared with the results using the lookup
process. The table values can then be adjusted to produce the
correct known response.
[0079] FIGS. 9 and 10 are schematic views showing how lookup tables
may be used to generate multipliers for transforming polyenergetic
data sets into any number of monoenergetic data sets, each one free
of beam hardening. Monoenergetic images can then be transformed
into a mapping of material properties such as effective atomic
number or electron density.
Generation of Lookup Tables for Determining Z.sub.effective or
Other Material Properties
[0080] The conversion between monoenergetic sinogram data (i.e.,
g.sub.ENERGY, which could be, for example, g.sub.160, g.sub.40,
etc.) and monoenergetic image data (i.e., I.sub.ENERGY, which could
be, for example, I.sub.160, I.sub.40, etc.), and material
properties, is a mathematical function. In the preferred
embodiment, this function is reduced to a tabular form. The values
within the table can be derived from the known physical properties
of materials or from empirical analysis of data. In the preferred
embodiment, the table is empirically derived. For example, samples
of varying mean atomic number (Z.sub.effective) are scanned. The
ratio of two monoenergetic images (e.g., I.sub.160, I.sub.40) is
used as an index into a table which returns the Z.sub.effective
value. Thus, to generate the table values, the image ratio and
Z.sub.effective value are recorded for a sample material. Data from
six sample materials is collected and a best fit function
Z.sub.eff(Image Ratio) is determined.
Modifications
[0081] It will be appreciated that still further embodiments of the
present invention will be apparent to those skilled in the art in
view of the present disclosure. It is to be understood that the
present invention is by no means limited to the particular
constructions herein disclosed and/or shown in the drawings, but
also comprises any modifications or equivalents within the scope of
the invention.
* * * * *